PS/2 allows for more key presses because that is the way the PS/2 spec was written; the USB spec is only written to allow a maximum of 6 plus modifier keys (Alts and Ctrls) for a max of 10 if you want to look at it that way.

Also just FYI PS/2 to USB converters (as well as USB to PS2 converters) will NOT work with Deck products. Therearemany threads with more details, just in short they will not work.